1. Field of Endeavor
The present disclosure relates to power plants or steam generators, and, more particularly, startup and shutdown preparations of power plants or stream generators or boilers, specifically to those power plants, which are subject to frequent shutdowns and startups.
2. Brief Description of the Related Art
Certain steam generator or boilers used in power plants are subject to frequent shutdown and startup. For example, concentrated solar power plants that have dependability on the solar energy to operate during daytime while shutting down in night (referred as shutdown period).
Such concentrated solar power plant uses solar boilers for producing steam to operate steam turbine in turn producing electricity by utilizing generators. Generally, a solar boiler may, apart from various other components, include an evaporator section and high temperature components, such as superheater section or reheater. The evaporator section produces steam and supplies it to the high temperature components, such as the superheater section, which superheat the steam to supply superheated steam for operating the steam turbine. Each of the evaporator or superheater section includes various fluidically connected panels having various fluid-carrying tubes vertically arranged between respective top and bottom horizontal headers, which are thick walled and generally insulated. These panels are heated by focusing sunrays thereon, in turn heating the fluid to be utilized for producing electricity.
During normal operation, the high temperature components, such as the panels of the superheater section, reach to its maximum temperature, and during the shutdown period it loses heat and reaches at relatively lower or higher residual temperature than that required for starting up the power plant in the morning. Specifically, the tubes in the superheater panels reach ambient temperatures as they lose heat to ambient air. Unlike the superheater tubes, the thick walled and insulated headers or manifolds of the superheater may not lose heat to ambient air. Therefore, during shutdown, the headers or manifolds are at relative lower or higher residual temperature than required for startup, and are generally at higher temperature as compared to the ambient temperature tubes during shutdown. During daytime, while normal operation of the power plant, the temperature of steam and the superheater panels increase from the superheater component inlet sections (upstream) to superheater outlet sections (downstream), keeping the superheater outlet sections at higher temperature, and the superheater inlet sections at relatively lower. Therefore, even after losing some heat during the shutdown period, the superheater upstream components remain at relatively lower residual temperature as compared to the superheater downstream components. At such condition, starting up of the power plant without any preparation may lead various problems such as fatigue damage of superheater panels, particularly headers and manifolds, due to substantial difference between temperature of steam coming from the evaporator section and temperature of the headers and tubes of the superheater panels. This temperature difference between the steam and header may generate through wall temperature gradients in the header causing thermal stress.
Accordingly, there exists a need to balance the thermal stress and improve the fatigue life of superheater upstream and downstream components of the superheater to increase overall life of the power plants.